The present invention relates generally to an electrochemical sensor for the detection of carbon monoxide and, more particularly, to the construction, operation and method of manufacturing a carbon monoxide sensor having a gas diffusion capillary transport construction for controlling the gas entry to the sensor, a water permeation barrier for controlling the transport of water vapor from the water reservoir to the main sensor cell, and a method of compensation for changes in sensor CO sensitivity caused by humidity changes.
Electrochemical sensors for toxic and combustible gases sense the presence of the target gas by either an electrochemical oxidation or reduction process at a catalytic electrode surface. This process either liberates (oxidation) or consumes (reduction) electrons which are supplied to the electrode by the measurement circuit. The current represented by this flow of electrons is the output of the sensor and it is directly proportional to the number of target gas molecules reacting at the electrode.
The electrode (where the target gas reacts) is generally referred to as the “working electrode” and it is partnered with a second electrode which is not exposed to the target gas. The second electrode is generally referred to as the “counter electrode” and it is separated from the working electrode by a material that will support the flow of chemical ions, but will not support the flow of electrons, namely, an ion conducting material. The counter electrode is also connected to the measurement circuit and undergoes electrochemical reactions so as to balance the current associated with the working electrode.
The ion conducting material is referred to as the “electrolyte” and typical examples include sulphuric acid and salt water. The electrochemical reaction at the working electrode also produces ions, in addition to the production/consumption of electrons, and these ions diffuse through the electrolyte to the counter electrode. Here they consume/liberate electrons to balance the charge flow in the system and ensure that there is no net accumulation of charge on either electrode.
The rate of electrochemical reaction can be controlled by the voltage differential between the working electrode and the electrolyte, so sometimes a third electrode is added to the structure (referred to as the “reference electrode”) to help control this potential. However, the design of sensors used in most carbon monoxide (CO) detectors for residential use is a two (2) electrode sensor with just working and counter electrodes.
Although there are many types of ion conduction mediums, only a few function well at room temperature. The first group of electrolytes are based on ion containing liquids (often aqueous but not always) such as acids or salt solutions. The second group are the ion conducting plastics which is epitomized by the material sold under the trademark NAFION, produced by DuPont.
The sensors mentioned above all require the presence of water in the electrolyte in order to function and there are two strategies to ensure its presence.
The first strategy, which works well with sensors based on mineral acids such as sulphuric acid, relies on the hygroscopic nature of a concentrated acid to draw water into the sensor when the external atmosphere is humid. In contrast, when the external atmosphere is dry, the electrolyte will lose water, increasing the concentration of the acid. The reduction in water content of the acid reduces the water vapor pressure above the acid and at some point this comes into equilibrium with the water vapor pressure in the surrounding atmosphere. When equilibrium is reached the electrolyte stops loosing eater and provided that the atmosphere is not completely dry, the electrolyte retains sufficient water to function. However, as the atmospheric humidity swings up and down, the strength of the acid in the electrolyte changes and in extreme cases of high humidity, the increasing volume of the electrolyte (caused by water absorption) can cause the sensor to burst. In addition to these problems, acid based sensors require the use of noble metal internal parts (such as gold or platinum wires) making them expensive and there are always corrosion problems.
The second strategy does not rely on atmospheric moisture to replenish the water in the electrolyte. Instead, the sensor incorporates a water reservoir to keep the electrolyte in good working condition. Water is continually lost from this reservoir to the outside atmosphere through the gas entry hole (the hole through which the target gas can get from outside the sensor to the working electrode) and it is usually this loss of water that limits the lifespan of this type of sensor. However, provided the entry hole is small, the water vapor pressure inside the sensor is almost constant, providing constant and repeatable water content for the electrolyte.
It is therefore desirable to provide a carbon monoxide sensor which can improve the lifespan of the sensor by controlling the gas entry to the sensor, by controlling the relative humidity inside the sensor, and by compensating for changes in sensor CO sensitivity due to humidity changes.